An effluent to be purified can be introduced into a high-loaded activation stage, operated as an adsorption stage, then passed through an intermediate clarifier with sludge offtake and subsequently introduced into a low-loaded stage to which a final clarification stage with sludge offtake is connected.
Optionally, anaerobic microorganisms can work in the adsorption stage and the sludge can be maintained in the initiation phase by means of the sludge offtake from the intermediate clarification stage.
Nitrification and subsequently denitrification can be carried out in the low-loaded stage. Additionally, sludge from the intermediate clarifier can be introduced as recycled sludge into the adsorption stage and sludge from the final clarification stage can be introduced as recycled sludge into the low-loaded stage, excess sludge being drawn off and thickened.
The "adsorption stage" signifies a biological stage that is equipped as a high-loaded activation stage, e.g. with the following parameters:
Volume loading LV=2-20 kg BOD.sub.5 /m.sup.3 .multidot.d, PA1 Dry substance content DS.sub.V =1-10 kg/m3, and a PA1 Dry sludge loading L.sub.DS equal to or greater than 2 kg PA1 BOD.sub.5 /kg DS.multidot.d PA1 preferably L.sub.DS =0.15 kg BOD.sub.5 /kg DS.multidot.d
In this respect the invention starts from the features known in the practice of A-technology (adsorption waste treatment, see German patent No. 26 40 875).
The low-loaded stage can be an activation tank but it can also be a trickle filter stage plus activation tank. As a rule, but not necessarily, one operates with separation of the bacterial colonies between the adsorption stage and the low-loaded stage, and this particularly in such a way that sludge from the low-loaded stage is not fed back into the adsorption stage.
The term "stage" also covers aggregates of several tanks or basins, several trickle filter installations and the like, which however are operated in the same way or practically in the same way in individual stages.
Phosphorus elimination has already been achieved with the known generic process (German Offenlegungsschrift No. 26 40 875). It is of the order of a phosphorus content amounting to 1 to 4% of the dry weight of the withdrawn sludge, which is regarded as the conventional phosphorus elimination level of A-technology.
If greater phosphorus elimination levels are required, other techniques are used in practice. The so-called Phostrip process is known (U.S. Pat. Nos. 3,236,766; 4,042,493; 4,141,822) in which part of the recycled sludge is passed through an anaerobic tank, where the phosphates are stripped out by washing with water. The phosphorus is removed from this washing water by precipitation with lime.
In addition, the so-called Bardenpho process is known (U.S. Pat. No. 3,964,998). This relates to a process for the biological removal of nitrogen and phosphorus. The process operates with five stages connected in series in the form of different types of tanks or zones of tanks. The anaerobic conditions necessary for the elimination of phosphorus are present in the first stage.
As the recycled sludge is also conducted into the first stage, care must be taken that it contains no nitrate. The nitrates that are formed in the first aerobic tanks are conveyed with the feedback into the first anoxic tank where they are denitrified, after which further movement into the aerobic stage takes place. The feedback level amounts to about 400%.
After this circulation the mixture of effluent and activated sludge is conveyed into the second anoxic stage where the remaining nitrates are denitrified. In order to avoid anaerobic conditions in the final clarification, to improve the capacity of the sludge to settle, and to remove small adhering nitrogen bubbles, a second aeration stage is necessary. In an aerobic procedure, of course, dissolved oxygen is present in the effluent that is to be treated. Under anoxic working conditions, however, although dissolved oxygen can no longer be found in the effluent to be treated, nevertheless oxygen is present is combined form as NO.sub.3 -- or as NO.sub.2. Anaerobic conditions are present when neither dissolved oxygen not combined oxygen in the form of NO.sub.3 or NO.sub.2 is present.